Good Morning, I hope everyone had a great weekend, I sure did (not Classic-Computer related so I won't bore anyone with the details)... Subsequent to my first post about my Prime PSU, Don Maslin sent me a howto on repairing switchers, that was written by Keith Lofstrom. The howto is fairly comprehensive, but does assume that most repairs are taking place due to bad PSU design. I'm not sure mine failed due to bad design. It was dusty, and after I removed it, I found that the 2455 cabinet has an interestingly-design set of airflow guides that's above the cage; it's basically a sheet of thick plastic with holes in it, so that the dual-squirrel-cage blower dumps air into the cage through these holes. While there was some airflow, once the PSU was out, I could see that many of the holes were clogged up with dust and lint, and thus I think the PSU simply overheated inside the cage. One additional data point that I thought I'd mentioned, but may not have, is that: The system was up and running fine, no evidence of any problems. I shut the system down that night, and then the next day when I turned it on, I got nothing. I did not see any evidence that the PSU briefly came on and then died; it simply never came on. about switcher operation, and from Keith's howto, it seems like something failed that is part of getting the secondary supplies "jump-started" to an up-and-running state. So, as long as it was powered up, it would continue to operate. But once I turned it off, the portion of the PSU (something between the primary and secondary supplies? Something like that big power resistor, maybe?) that gets the secondaries going, couldn't get them going. So, has anyone else been in a similar position? Do I have a fair chance of simply replacing the parts I've found that failed, reinstalling it, and being successful? There is a 5v adjustment that needs to be made before bringing the system up all the way, and while I hate to use the system itself as a dummy load, I don't have the Prime bus pinout, so I can't easily determine what's what. One of the things Keith discusses in the howto is about parts substitutions. Most of his comments are w/r/t transistors and the like, and he suggests using as replacements devices that have voltage and current capacities that are increased over the part that failed. He goes on to say you need to test each such substition with a curve tracer before finalizing each such substition. However, I don't think any semiconductors failed for me. He says that in a good design, there are some power resistors protecting the switcher transistors, and from my examination to date, it appears that Prime did in fact put these in the design. I'm going to remove the other 3 of the 4 that are in that area to be sure whether any did or did not go open-circuit, as there is significant thermal discoloration of the underside of the PCB in that area. How reasonable a course of action is this? What's likely to happen should a problem still be present? I'm thinking that worst case, the parts that failed will just quickly fail again, putting me back at square one. As always, thanks in advance for any help y'all can give me! regards, -doug quebbeman